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Here on Earth, Mars and most of others if not all terrestrial planets in the universe, iron is their most common metallic element because iron is in general the most common metal in the universe.

Here on Earth, a desert's sands are mostly 1 of these 3 colors:

Beige: By far the most common color in Earth's deserts and probably in the universe too. Almost all deserts are mostly some shade of beige, for example, the Sahara is mostly beige except for a few areas

Reddish orange: The best example would be Australia. Their deserts get so red that even martian soil falls short in comparison... Well in reality mars is mostly brown. You can even clearly see how Australia is way redder than any other landmass just by looking at it from space, so much redder in fact that it's often called "the red continent". The orangey sands are because australian soil contains some iron and in contact with the air (oxygen) it simply rusts.

Black: Black deserts are not very common and they are all quite small since they are just the product of old volcanic eruptions that left behind a lot of black basaltic rocks around them that eventually got wind-eroded into sand. An amazing example is Waw An Namus, it's big enough of a place that it can easily be seen high up from space as a black dot in the middle of the Sahara, you can easily find it just by view if you search it on google maps/earth.

Now, what about blue-green deserts? These would be the result of, instead of iron rich soil, copper rich soil. As iron, copper rusts in contact with oxygen but copper rust is not red as iron's is blue-green! Here on Earth it seems there's not enough copper for us to get these amazing looking hypothetical deserts... But hey, at least we got this! Yes that is not photoshopped, the surreal place is just a 100×50 meters area in the middle of the Philippines called "kaman utek" and yes, its soil looks blue because of copper oxide. As far as I know, this place is unique and there's nothing quite like it here on Earth, might be wrong tho.

I believe this is proof enough that if a planet's soil is abnormally copper rich and has some oxygen in its atmosphere, huge blue deserts are possible just like Mars is pretty much a single huge brownish-red desert because of iron oxide. But what I don't know is (and here comes the question). Is it even possible in the real universe for a planet to have that much copper? Even if we are talking about 1 in a billion. And if not, at least some decently sized patches of blue sand dunes?

Thanks in advance!

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  • $\begingroup$ Huh, I went to look up the Waw An Namus on Google Maps, and was hoping to have a "Street View" node near it. Unfortunately they don't have one there, but the closest one is about 160 miles to the northwest - and I don't know, is that white sand? Because it definitely looks like snow, which I don't think they get in Libya. I have to assume that thing's got incorrect geo-tags on it, unless there's some snowcapped mountains in the Sahara I've never heard of... $\endgroup$ Jul 15 at 18:29
  • $\begingroup$ @DarrelHoffman It does snow very rarely in the Sahara, so you probably saw white sand. geographyrealm.com/often-snow-sahara-desert/…. $\endgroup$ Jul 15 at 19:38
  • $\begingroup$ There are mountains bordering the Sahara that are high enough to get snow, though I don't believe they hold it year round. The Atlas Range, for instance. Couldn't tell you if they're visible from Libya, though... $\endgroup$
    – Zeiss Ikon
    Jul 15 at 19:40
  • $\begingroup$ @M.A.Golding Yeah, this also looks like it's in the middle of a town though, and there doesn't appear to be a town in this area: see link $\endgroup$ Jul 15 at 20:37
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    $\begingroup$ Copper oxides aren't generally green though? Cuprous Oxide (CuO) is red, Cupric Oxide (Cu2O) is black. While Copper Peroxide (CuO2) is green it's not stable, and dry (anhydrous) Copper Chloride (CuCl2) is yellow - it needs water to turn green. Copper Carbonate Hydroxide (CuCO3(OH)2) is green, Copper Sulphate (CuSO4) is blue... but only when hydrated. So... not certain Copper is the right way to go for a green desert. $\endgroup$
    – Corey
    Jul 16 at 1:38

10 Answers 10

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Nickel compounds.

nickel oxide source

Nickel is almost as common as iron in the universe; I refer you to the fine table Dutch posted. The core of the earth is made of nickel and iron and that is what many metallic asteroids are made of as well. Nickel oxide (depicted) is an awesome green but many of the nickel compounds have the blues and greens you want.

http://wwwchem.uwimona.edu.jm/courses/nickel.html#:~:text=Nickel%20oxide%20is%20a%20powdery,nickel%20hydroxide%2C%20carbonate%20or%20nitrate.

Nickel Compounds

Nickel is known primarily for its divalent compounds since the most important oxidation state of the element is +2. There do exist however certain compounds in which the oxidation state of the metal is between -1 to +4. Blue and green are the characteristic colours of nickel compounds and they are often hydrated.

Nickel hydroxide usually occurs as green crystals that can be precipitated when aqueous alkali is added to a solution of a nickel (II) salt. It is insoluble in water but dissolves readily in acids and ammonium hydroxide.

Nickel oxide is a powdery green solid that becomes yellow on heating. It is difficult to prepare this compound by simply heating nickel in oxygen and it is more conveniently obtained by heating nickel hydroxide, carbonate or nitrate. Nickel oxide is readily soluble in acids but insoluble in hot and cold water.

I could imagine a world that had a late bombardment of nickel-rich meteorites might have a surface enriched for nickel compounds.


Olivine.

https://en.wikipedia.org/wiki/Olivine

olivine

The mineral olivine (/ˈɒl.ɪˌvin/) is a magnesium iron silicate with the chemical formula (Mg2+, Fe2+) 2SiO 4. It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle,[8] it is a common mineral in Earth's subsurface, but weathers quickly on the surface.

Olivine gets its green from magnesium and iron. It is not rare. I figured there must be sand somewhere that was green because it was made of olivine. Yep.

green sand beach

https://amazing.zone/green-sand-beach

The aptly named green sand beach in Hawaii gets its color from olivine.

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    $\begingroup$ Nice. I did quick survey of common copper compounds and there aren't many green ones, especially in arid conditions. Your options are better IMO :) $\endgroup$
    – Corey
    Jul 16 at 1:40
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    $\begingroup$ nickel compounds also means many of your desert will glow under ultraviolent light. $\endgroup$
    – John
    Jul 16 at 4:41
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    $\begingroup$ Did a quick Google Street View tour of Green Sand Beach, there's dozens of panos there, and none of them are anywhere near that green. I suspect that photo might be doctored a bit to exaggerate the effect. That or maybe there was an algae bloom nearby that made the sand greener than normal that day... $\endgroup$ Jul 16 at 13:57
  • $\begingroup$ @DarrelHoffman green sand beaches really is that green, note there are four green sand beaches if you search you can find pictures of people holding the sand in their hands. $\endgroup$
    – John
    Jul 16 at 21:00
  • $\begingroup$ @John It's the same beach, the one in Hawaii, you can tell by the rock formations. No shortage of photos and panoramas on Google Maps of this location. The sand looks greenish in all of them, but not nearly as green as this photo, which just looks like they cranked up the saturation. $\endgroup$ Jul 16 at 21:13
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It seems to be difficult.

Abundance-wise, copper is about 3 orders of magnitude less abundant than iron if we look at the solar system

enter image description here

This means that, on average, for every mole of copper you will have about 1000 moles of iron.

To make things worse, copper (8.96 $g/cm^3$ @RT, 8.02 $g/cm^3$ when liquid) is also more dense than iron (7.87 $g/cm^3$ @RT, 6.98 $g/cm^3$ when liquid), this means that in a differentiated planet iron will float above copper, making copper even more scarce on the surface, even if you started with equal initial abundance.

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    $\begingroup$ I feel a statistical improbability in RL should not have implications for SF worlds built, as long as science tells us it is possible to have certain improbable phenomena. There could be diamond asteroids, some contain lots of gold.. why would a planetary crust with excessive copper concentration be non-existent ? Whether that planet would become green or blue depends on the oxigen in the atmosphere. $\endgroup$
    – Goodies
    Jul 15 at 10:22
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    $\begingroup$ @Goodies, because in the scale of the universe an asteroid is small enough to be a statistical fluctuation, while a planet is big enough to start enforcing the law of the big numbers. $\endgroup$
    – L.Dutch
    Jul 15 at 10:26
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    $\begingroup$ Still, you can't exclude these things, imho.. question here is "is it possible" to yield a high concentration of copper in a planetary crust, visible as green or blue teinted deserts. It may be sufficient to have a few large high-copper content asteroids crash on the planet, or merge with the developing planet, in an early stage. $\endgroup$
    – Goodies
    Jul 15 at 10:32
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    $\begingroup$ If there were some intelligent process that concentrated copper on one planet (crazy aliens bringing in gigatons of the stuff), what happens when 100 million years later they are gone and their solid copper spacechicken coops have eroded? Does that planet get blue deserts? $\endgroup$
    – John O
    Jul 15 at 13:45
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    $\begingroup$ @Goodies Keep in mind the relative abundance of copper to iron has been found to vary from star to star (see kencroswell.com/Copper.html), with later forming stars more enriched in copper. Plus copper abundance in the interstellar medium could be "clumpy" after it's ejected from a supernova, so that some stars get a larger dose of copper in the gases & dust from which they formed. $\endgroup$
    – RobertF
    Jul 17 at 19:40
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Perhaps it came from outside:

As L.Dutch has correctly mentioned, so much copper in the surface of the planet may not be so common only atributed to the formation of the planet. But what about imported from outside in a later stage?
I mean: Copper has been discovered in meteorites:

https://www.sciencedirect.com/science/article/pii/S0032063320303056

In this paper, we report on abundance and mineral occurrences of copper in H chondrites, identify Cu carrier minerals and interpret their distribution in the context of processes that must have affected parent bodies. This leads us to suggestion that some parts of S-type asteroids contain Cu in a form and amount that would satisfy requirements of potentially economically and environmentally more efficient exploitation.

So, you could design in the history of the planet an event where many asteroids with copper arrived and populated the surface with the mineral.

Or the volcanic activity helped:

Yes, the copper is normally trapped inside the planet, not in the surface. But volcanic activity can consolidate big copper deposits within them:

https://www.sciencedaily.com/releases/2015/02/150209113220.htm

Copper forms in association with volcanoes such as those around the Pacific Ring of Fire.

Professor Blundy and colleagues speculate that copper deposits are forming beneath many active volcanoes today, including the Soufrière Hills volcano on the tiny Caribbean island of Montserrat that has been erupting since 1995.

And then, after a few centuries, you just need an earthquake or a continental drift fracture or (why not?) another meteorite to expose those big copper deposits and create your blue desert.

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Chemically, one tends to find copper in close association with silver and gold. I've been told that once upon a time (even as late as the 1960s to 1970s), payrolls for the Anaconda copper mine in Montana (at the time, the largest copper mine on Earth, both by production and by area/volume) were met solely from the silver and gold produced as byproducts of the copper production that was the reason for the mine's operation.

As noted in another answer, however, copper is relatively scarce compared to metals like iron, magnesium, aluminum, and the alkali metals -- about a thousand times scarcer than iron.

One way around this would be for something to have consumed most of the iron in the planet's crust at some point in its history (perhaps nano-mining by a race of Progenitors who colonized the galaxy a billion years ago and then vanished). Most of the other more common metals have either white or only weakly colored oxides. Combine that with a long period of erosion, the sort that produces alluvial gold deposits on earth, and you could get low-lying areas, originally river flood plains or deltas, that are enriched with copper compounds, but with the whole crust iron poor (except in hotspot volcanic regions, where eruptions would replenish iron from mantle rocks).

Now, a transition from wet to dry would transform those flood plains into flat, arid silt -- which, instead of being colored beige or orange by iron, could be green to blue due to the alluvial enrichment of copper.

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Your deserts could be blue not because of copper dust, but because of algae.

More specifically, a particular species of diatom native to your planet, which is oddly resistant to biological or physical degradation, and has a particularly rich blue color from its phycocyanin-like pigments. Over billions of years, they could accumulate to be a major component of the dusts and silts of your planet, and would blow with the winds to form vast, colorful dunes in your deserts.

Pigments do tend to photobleach over time with exposure to light, and ultraviolet light in particular. Perhaps there's enough of a constant production of algae to cover the faded sands, or your species resists this effect due to some oddity in its silicate shell, or in the mixture of accessory pigments in its cytoplasm.

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  • $\begingroup$ That sound like a good idea! I'll keep that in mind $\endgroup$
    – DeMooniC
    Jul 15 at 21:27
  • $\begingroup$ If it's biomass rather than mineral, though, would it actually be a desert? $\endgroup$
    – kaya3
    Jul 16 at 12:14
  • $\begingroup$ I was thinking the diatoms would be dead - like the fossilized ones you find in "diatomaceous earth" on this planet. I have no idea if such dead diatoms could maintain any kind of pigment over the timespans needed to accumulate enough to make the deserts blue...the "nickel" comment above is probably far simpler and more likely. $\endgroup$
    – Ed Beaty
    Jul 18 at 6:00
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This is more complicated than it sounds. Most red sand or soil is a result of iron (III) oxide. Sesquioxide compounds like this are an exceedingly common component of soil due to a handful of specific aspects of Earth’s geochemistry. Copper sesquioxide is currently only theoretical as an independent compound, and the lower order oxides are not only not common in soils, but not actually blue or green either.

So we need to look at a different geochemistry from that of Earth for this to work.

Possibilities that come to mind include:

Basic copper carbonates.

Specifically, malachite and azurite. Both can occur in Earth-like geochemistries, but the conditions required to get a desert colored with them are not readily present here on Earth. Malachite ranges from a deep forest green to a pale sea-foam green, while azurite is a dark blue. In theory, on an especially arid planet, you could have a desert form where the ‘sand’ is made of these two minerals instead of silica like is common here on earth, and it should have a lovely blue-green color if viewed from space.

Nickel and/or chromium oxides

Nickel (II) oxide and chromium (III) oxide are both a rather lovely shade of green, and both can behave in the ways required to color a desert. They would both require some shifts in geochemistry to become prevalent enough to achieve such a result though, most notably an atypically low availability of iron in the planetary crust. You can’t get blues this way, but seen from space the greens could be easily mistaken for earth-like plant life from a limited cursory inspection.

Certain hydrated metal salts

Hydrated copper (I) chloride is a nice greenish color (actually pretty close to the above mentioned chromium (III) oxide), copper (II) chloride dihydrate and copper (II) sulfate pentahydrate are the classic ‘copper blue’, and a handful of other ionic copper salts also produce similar colors when there is water present in the crystal matrix. The case is similar for iron (II) salts with water in their crystal matrix, which are a rather peculiar sea-foam green color. For this to work, liquid water needs to be relatively rare, but water needs to be chemically available such that the salts can form and don’t dry out, as the anhydrous forms are invariably differently colored.

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A couple thoughts:

In the book The War With Earth, a gas giant fried by a nearby supernova is rendered down such that the metallic core separates as it cools into distinctive layers based on mass, such that each element forms a semi-pure layer. In such a scenario, all the copper would be deposited in a layer, and if the upper layers of such a body were stripped off, the resulting surface would be nearly pure copper. Given time, and atmosphere, and wear, this could mean a world/star system starting out with something like this could be very copper-rich.

Slightly more realistically, if the planet's life is highly dependent on copper, then the organisms could be sequestering all the possible copper on the planet - digesting minerals and so on. Now if you have a die-off or biologically deposited layer of dead organisms, the surface that becomes your desert might be highly enriched in copper compounds (the copper equivalent of diatomaceous earth).

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Copper oxides are red (Cu2O a.k.a. cuprite) or black (CuO). I work with cuprite and it can also look black. Red oxide paint primer is based on copper oxides, as an example of how they look crushed up small; the residue from polishing cuprite is a very similar colour to rust.

However copper carbonates (basic, chlorides, sulphates and acetates are blue or green. Some of my cuprite has greenish inclusions. If you can design enough copper into your world, these shouldn't be too hard to explain. The carbonate, for example, forms under (Earth) ambient conditions, as does the chloride in the presence of common salt.

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Just like the oxygenation of the Earth's oceans, caused by the evolution of photosynthesis, lead to massive banded iron deposits here on Earth, it's possible that some form of microbe could cause and maintain concentrations of large deposits of green copper compounds over the course of a few billion years. In the absence of plate tectonics, such deposits might not be destroyed by natural processes, so even a very slow acting biological process could have a huge effect over time.

A combination of microbes, plants and even animals, could be used to explain concentrations of almost anything that might otherwise be unlikely due to pure geophysics.

We use bacteria to extract copper from ores, we're an animal, using bacteria to concentrate copper compounds at the surface of the planet, that would otherwise get mixed into the mantle at levels near average solar system abundance.

See also: https://www.nature.com/articles/ismej200775.

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Just to give a different view - note that everything on your planet may just appear to be green(ish), not because of the reflective/absorptive properties of the material (surface) you are looking at, but because the light reaching the material is not pure white, eg.:

  • if star output is blue-ish (can't really go for full green), instead of being distributed across all visible spectrum, everything you look at will be skewed toward that color (imagine someone whose shirt is bright red under the noon sun - under blue disco reflector light it would look different color)

  • if planet atmosphere blocks part of the light spectrum (can also be combined with above), the perceived color with also change (note how big fires or vulcanic eruptions change the colors, also related but different: photography taken at noon vs. sunrise/sunset)

You can combine both with all the previous answers (so you can for example have combination of blueish star + yellowish sand make the sand appear greenish etc).

Also note that color perception in a biology thing. Remember when those first LED light come out (and before them, CFLs), how they had "cold white" temperature, eg. were bluish. Yet when you looked at peace of paper after few minutes of lights being on, you would swear the paper looked white and not blueish (same and more pronounced effect, but in opposite direction, can be seen with orange/reddish candle light). It is because you brain has performed automatic white-balance correction according to its feeeling / memory of how paper should be looking.

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